PREFACE XI 



to take his class into the field. Under these circumstances he 

 should constantly impress upon the minds of his pupils the inade- 

 quacy of all schemes and systems to embrace the great facts of 

 nature, and should encourage them to observe for themselves, 

 testing what they read by what they see. 



In preparing this book, I have of course availed myself of 

 material wherever it was to be found, but I wish to acknowledge 

 my special obligations to the text-books of Dana, Le Conte, 

 Geikie, Green, Prestwich, Credner, Kayser, Neumayr, Koken, de 

 Lapparent, and Jukes-Brown. From the last-named writer is 

 taken the arrangement of the Dynamical Agencies, which expe- 

 rience in the class-room has led me to consider as the best. 

 Besides these general works, I have received great help from 

 monographs and special articles by many writers, particularly 

 from those by Clark, Cross, Dale, Dean, W. M. Davis, Gilbert, 

 Harris, Kemp, Russell, Van Hise, Walcott, Willis, Weed, and 

 others. 



I take sincere pleasure in acknowledging the extremely kind 

 and ready assistance which many fellow-workers in all parts of the 

 country have granted me with unsparing liberality. Mr. Walcott, 

 Director of the United States Geological Survey, has been espe- 

 cially kind in this respect, and has allowed the fullest use of the 

 Survey's fine collection of photographs. The liberal way in which 

 advantage has been taken of this permission is to be seen in the 

 many illustrations in the following pages marked (U. S. G. S.), all 

 of which were made from the Survey photographs. Many other 

 members of the United States Geological Survey have spared no 

 pains to help me in the work of compilation, with advice, infor- 

 mation, papers, drawings, photographs, and every other means in 

 their power. To these gentlemen my obligations are very great, 

 and to Messrs. Walcott, Cross, Emmons, Gilbert, Hill, Weed, and 

 Willis I wish to express my cordial thanks for many acts of kindly 

 and most valuable assistance. 



Professor J. F. Kemp was so kind as to send me the advance 

 sheets of his " Lecture Notes on Rocks," of which extensive use 

 has been made. Mr. A. Smith Woodward, Mr. Agassiz, Dr. Bash- 



